In this case I believe it was because:
When Galaxy 17 was launched it was suppose to replace G11 at 91 degrees west, but SBS6 was getting weaker and Galaxy 11 was ok at the time so they temporarily replaced SBS6.
Now Horizons 2 (originally intended to replace SBS6) is being launched and it will take its planned position at 74 degrees West and Galaxy 17 will move to its originally planned position at 91 degrees West.
The reason to move the satellite is probably due to each satellites capabilities, and what it was designed to do.
-EDIT-
Horizons 2 is a Ku-Band only satellite (20 Ku-band transponders), SBS 6 was a Ku Band only Satellite.
Galaxy 17 is a C/Ku Band Satellite (24 Ku-band and 24 C-band transponders), Galaxy 11 is a C/Ku Band Satellite.
It makes sense to replace like satellites.
-EDIT-
Horizons 2 has a CONUS and East Coast only beam

(I am on the West Coast, so I hope they turn the satellite upside down

)
Source:
Jsati Horizons-2